1. Field of the Invention
This invention relates to an automatic transmission control system for an automobile.
2. Description of the Related Art
Automatic transmissions for automotive vehicles, which typically includes a torque converter and a transmission gear mechanism comprised of a planetary gear mechanism, are automatically shifted into desired gears by selectively coupling or locking and uncoupling or unlocking a plurality of friction coupling elements, such as a forward clutches and a 3-4 clutch or a 2-4 brake for locking a specific rotary element of the planetary gear mechanism, to switch the power transmission path of the transmission gear mechanism with the result of providing desired gears. Some of this type of automatic transmissions, while two specific friction coupling elements are unlocked, another specific friction coupling is locked to cause a specific gear shift.
Such an automatic transmission performs feedback control to bring a turbine speed of the torque converter into agreement with a target speed during the gear shift, causing a smooth increase in the turbine speed toward a speed to be attained at an end of the gear shift. In particular, in the case where a specific gear shift is caused through replacement of locked and unlocked friction coupling elements, after having unlocked one of the two specific friction coupling elements, control of a locking pressure is performed to allow another specific friction coupling element to cause slippage so as to bring the turbine speed into agreement with a target speed during the gear shift. At the moment at which the turbine speed reaches the shift-end speed, while another specific friction coupling element is locked, the other specific friction coupling element is completely unlocked to achieve the gear shift. As an example, in cases where the automatic transmission is shifted to a fourth gear by means of locking both 3-4 clutch and 2-4 brake and to a first gear by means of locking a forward clutch only, a 4-1 (fourth to first) gear shift is achieved by unlocking these 3-4 clutch and 2-4 brake and locking the forward clutch. In this instance, with the state wherein, while either one of these 3-4 clutch and 2-4 brake is unlocked in advance, another one of them is left locked, the forward clutch is unlocked after having increased the turbine speed through feedback control which is performed by controlling the locking pressure for the other friction coupling element, namely the 3-4 clutch or the 2-4 brake.
In cases where the specific friction coupling element to be locked during the 4-1 gear shift is the forward clutch which has a drum fastened to a turbine shaft as a torque input member to the transmission gear mechanism and a hub fastened to a specific gear element of the planetary gear mechanism as a torque output member from the transmission gear mechanism, locking the forward clutch is practically achieved by means of locking between these drum and hub. In such a case, in some mechanical structures of planetary gear mechanisms which impose a constraint on which one of these 3-4 clutch and 2-4 brake should be unlocked in advance of the other, there occurs a great error in angular acceleration between these drum and hum of the forward clutch during the feedback control of turbine speed, resulting in a sharp torque draw when the drum and hum of the forward clutch are locked. Specifically, because the turbine speed feedback control causes the forward clutch drum to increase its speed, it is preferred for the forward clutch hub to increase its speed following the increase in speed of the forward clutch drum. In addition, a locking shock is suppressed by increasing and bringing speeds of the drum and hub of the forward clutch near closer to each other and locking the the drum and hub of the forward clutch when these speeds brought into agreement with each other.
However, in some order of unlocking the two specific friction coupling elements, there are cases where, following the locking of one of the two specific friction coupling elements, the forward clutch hub reverses the direction of rotation or the direction of angular acceleration with respect to those of the forward clutch drum, or otherwise, even when the drum and hub of the forward clutch rotate in the same direction with an angular acceleration in the same direction, they increase a speed difference. When, in such a case, the drum and hub of the forward clutch, which serve as torque input and output members, respectively, enter into locking operation, the planetary gear mechanism generates such a great change in speed therein as to result in a torque draw which is led to a shock. This shock is mingled with the locking shock caused when the forward clutch is completely locked together, resulting in a double-step shift shock.